Variations in electrical impedance of the human tissue may be indicative of tumors, lesions and other anomalies. For example, U.S. Pat. No. 4,291,708 to Frei, and U.S. Pat. No. 4,458,694, and the article, “Breast Cancer Screening by Impedance Measurements,” by G. Pipemo et al., Frontiers Med. Biol. Eng., Vol. 2 pp. 111-117, the disclosures of which are incorporated herein by reference, describe systems for determining the impedance between a point on the surface of the skin and some reference point on the body of the patient. With the use of a multi-element probe, a two-dimensional impedance map of an organ such as a breast can be generated. The impedance map, describing variations in impedance along the tissue of the organ, can be used for the detection of tumors and especially malignant tumors.
One of the factors which influences the chances of curing a patient having a malignant tumor is early detection of the tumor. Therefore, although many tumors are detectable using prior art impedance imaging systems, it is continuously desired to improve the resolution of these systems to allow earlier detection of anomalies which may be malignant tumors.
U.S. Pat. No. 4,291,708 to Frei, mentioned above, and U.S. Pat. No. 5,810,742 to Pearlman the disclosure of which is incorporated herein by reference, describe methods of impedance imaging of an organ. A multi-element probe is placed on a surface of the organ and a reference signal is applied to the human body including the organ at a reference point far from the imaged organ. The resolution of the system depends on the resolution of a sensor of the probe and/or on the signal strength of the reference signal. When a digital sensor is used, increasing the resolution may be achieved by performing analog to digital conversion over the entire dynamic range. This, however, increases the cost and power consumption of the probe. Increasing the signal strength of the reference signal is not desirable for reasons of safety, patient discomfort and power consumption.
The blood volume of organs of the human body changes with the cardiac cycle (heart cycle) of the human. The detection of the changes in the blood volume is referred to as plethysmography. Plethysmography is commonly used to determine the phase of the cardiac cycle, as described in U.S. Pat. No. 5,615,672, the disclosure of which is incorporated herein by reference, and in pulse oximeters which measure blood oxygen saturation.
UK patent application GB 2,138,148, the disclosure of which is incorporated herein by reference, states that impedance changes which are correlated with the cardiac, respiratory or other functions of the body, may be recorded, for example, for an evaluation of the stroke-volume of the heart and respiration and perfusion of the lungs. An article titled “In Vivo Imaging of Cardiac Related Impedance Changes”, B. Murat et al., the disclosure of which is incorporated herein by reference, states that it has been shown that the thoracic resistance variations during the cardiac cycle can be imaged by ECG-gated electrical impedance tomography (EIT). This article describes using EIT to image the blood flow to the lungs to detect abnormalities in pulmonary perfusion, such as pulmonary embolism.
U.S. Pat. No. 5,810,010 to Anbar, the disclosure of which is incorporated herein by reference, describes a method of cancer detection based on temporal periodic changes. The temporal periodic changes are suggested to be detected based on dynamic area thermometry (DAT), infrared sensing, MRI, or ultrasound.
Web pages available on May, 1, 2001 at www.dobimedical.com, the disclosure of which is incorporated herein by reference, describes a breast cancer detection system which measures transmission of red light through the breast. The system records the transient response to a pressure stimulus that initiates changes in blood volume and hemoglobin oxygenation.